Fuel cell electrodes comprising mixtures of silver, nickel, molybdenum and nickel-phosphorus alloys and method of making same



United States Patent FUEL CELL ELECTRODES COMPRISING MIX- TURES OFSILVER, NICKEL, MOLYBDENUM AND NICKEL-PHOSPHORUS ALLOYS AND METHOD OFMAKING SAME Henry J. Seim, Brookfield, James P. Murdock, West Allis, andTheodore L. Larson, Milwaukee, Wis., assignors to Allis-ChalmersManufacturing Company, Milwaukee, Wis. No Drawing. Filed Oct. 22, 1965,Ser. No. 502,590 5 Claims. (Cl. 136-86) This invention relates toimproved anode catalysts for activating fuel half-cell reactions withinfuel cells. More particularly, this invention dealswith fuel cell anodescomprising mixtures of silver, nickel, molybdenum and nickel-phosphorusalloys than electrochemically activate fuel cell fuels such as hydrogen,hydrazine, ammonia; and alcohols, esters, carboxylates, hydrocarbons,and carbonyl compounds of up to about 14 carbon atoms.

The term fuel cell, as used herein, refers to those electrochemicaldevices that convert the free energy of a chemical reaction directly toelectrical energy. Such devices are well known in the art and althoughthere are differences between various cells, a discussion of some oftheir common characteristics and problems will aid in the understandingof our invention. I

As is known, oxidation-reduction reactions are accompanied by thetransfer of electrons from the reductant to the oxidant. In individualfuel cells, the oxidation reaction and reduction reaction take place atspacially separated electrodes. At each electrode there occurs what iscalled a half-cell reaction. One electrode, called the anode, is thesite of the oxidation half-cell reaction. A reactant, referred to as thefuel, that is oxidizable with respect to some oxidant is supplied bysuitable means to the anode, and is thereat electrochemically oxidized.Oxidation of the fuel releases electrons to the anode. .At the otherelectrode, called the cathode, spaced apart from the anode by a suitableelectrolyte, the other half-cell reaction simultaneously takes place. Areactant called the oxidant, reducible with respect to the fuel, issupplied by suitable means tothe cathode, and is thereatelectrochemically reduced. This reaction takes up electrons from thecathode.

These two half-cell reactions result in the cathode tending to have adeficiency of electrons and the anode to have an excess. This tendencyis relieved by the transfer of charge electronically throughan externalcircuit connecting the electrodes, accompanied by the ionic transfer ofcharge through electrolyte. The current produced in the externalcircuitcan do useful work. Production of current will continue so longasfuel and oxidant are supplied and waste products exhausted.

nickel electrode base carry a deposit of a catalyst such, for example,as platinum group metals and oxides of the transition series elementsupon its surface.

We have discovered, as disclosed in co-pending United States patentapplication Ser. No. 430,726, filed Feb. 5, 1965, that when anodescomprise, in addition to nickel, a nickel-phosphorus alloy, the outputsgenerated by such electrodes exceed those obtained when only nickel isused.

The new and improved electrode we have discovered is disclosed inthe'followingspecification and claims com-- prising silver andmolybdenum in addition to nickel and nickel-phosphorus alloys. Silverhas heretofore nearly ex- The voltage of the individual fuel cell islimited by the theoretical free energy change (A F.) for the reaction atthe fuel cell operating temperature. The amperage of the cell isdetermined by the rate of reaction and the size of,

the cell. In practice, several individualfuel cells are couabout 95 C.,a catalyst is necessarily employed to bring the reactants to anactivated state. The energy input required to reach an activated state,i.e. heat of activation, I 9

partly determines the speed of reaction. Through a mechanistic bypass acatalyst brings about reaction with a l smaller heat ofv activation.

Nickel electrodes have found extensive use as anodes in fuel cellshaving a caustic electrolyte. In so-called low temperature operationefliciency demands that the porous clusively been used and regarded asan oxidant catalyst. We find that if molybdenum is co-precipitated withthe silver, nickel and nickel-phosphorus alloys a catalytic materialsuperiorto that disclosed in our earlier referred to patent applicationis obtained.

Hence, the general object of our invention is the provision of acatalytic fuel cell electrode.

' A further object of our invention is to provide a method of obtaininga fuel cell electrode of high surface area that comprises silver,molybdenum, nickel, and nickel-phosphorus alloys.

A still further object of our invention is to provide a fuel cellelectrode that exhibits extroardinary high outputs compared topreviously known nickel electrodes.

The methods by which the catalytic material, as it is referred tohereinafter, of our invention are attained will now be set forth indetail.

We have discovered that the catalytic material can be best obtained byreduction from an aqueous solution comprising a reducible source ofmolybdenum; a reducible source of nickel; a condensed polyphosphate thatcomplexes nickel ions (Ni++) in aqueous solution; and alkalihypophosphite; and a reducible source of silver. During the performanceof the reduction procedure the pH of the solution ought constantly to becontrolled so as to be above pH and preferably in the pH range of 10 to12. To achieve control, sufficient hydroxyl ions are added as a; alkalihydroxide, or better yet, as ammonium hydrox- 1 e.

The source of the molybdenum is preferably a water soluble molybdenumcompound leaving M0 in the +2 to 6 oxidation state. Examples of suitablemolybdenum compounds are M00 M001 and MoCl The quantity of molybdenumsalt present is adjusted so that thematen'al precipitated contains fromabout 2% to 9% molybdenum;

Likewise as" a sourceof reducible, silver, any water soluble silver.salt is suitable provided its anion does not interfere with thereduction processby itselfeither pre: cipitating or undergoing aredoxreaction. Again we find that the sulfate salt is quite suitable;-

The silver is reduced by the hypophosphite according to the followingreaction:, I

2 i)f-l-Ht Q s -E(' t )5 The silver is thought tobe formed in a finelydivided 1 alkali hypophose state perhaps even colloidal, therebyproviding a nuclei for the catalytic reduction of the nickel.

Of course once the reaction is underway some of the freshly formed Nican reduce the Ag+ to Ag thereby providing additional catalyst for thehypophosphite reduction of nickel.

- The quantity of silver salt present in the solution as expressed inmoles Ag ought not to exceed about half the quantity of nickel saltpresent expressed in moles Ni. For example, when using the sulfate saltswe find that the maximum useful amount of Ag+ is 0.55 mole per mole ofNi++.

Simultaneously, to the reduction of Ni, Ag+ and Mo ion some of thehypophosphite is decomposed. This decomposition is catalyzed by presenceof the freshly reduced nickel and silver proceeding as follows:

Therefore, the necessity of providing an excess of hypophosphite overand above that stoichiometrically required to reduce all the nickel,silver and molybdenum is apparent. If insufficient hypophosphite isprovided, no harm is done except that not all the nickel, silver andmolybdenum will be reduced.

We find that if 2.75 moles of (H PO are present per combined total molesof Mo ion, Ni++, and Ag' the reaction can proceed to completely depletethe solution of Mo ion, Ni++, and Ag+.

The function of the condensed phosphate is to complex the nickel so thatprecipitation of basic nickel salts is prevented when the pH is adjustedupward by the addition of hydroxyl ion. Therefore, to this end, anycondensed polyphosphate that performs the complexing function issuitable. Whatever condensed polyphosphate is used, it should be presentin an amount suflicient to complex all the Ni++ present and prevent theprecipitation of basic nickel salts. It is not a certainty from ourexperimental work and the literature that the molybdenum ion is actuallycomplexed by the polyphosphate.

When alkali pyrophosphate is used as the complexing agent, an amount ofat least 1.00 mole of P for each 1.45 moles of Ni++ is deemed theminimum pyrophosphate required for successful complexing of the nickel.Preferably 1.18 mole P O is present for each 1.00 mole Ni++. Whilepyrophosphates can of course hydrolyze and form orthophosphates, this isnot material to the performance of the process, because the nickel isreduced before the relatively slow hydrolysis becomes a serious problem.

Turning now to a specific embodiment of the practice of our invention,we shall outline the production of a batch of our catalytic material.

An aqueous solution having the following composition was prepared byadmixing 30 grams Na P O 25 grams NiSO -6H O; 2 grams AgSO 25 grams NaHPO -H O and 1 gram M00 per liter of water heated to about 60 to about 65C., although the reduction can be carried out satisfactorily attemperatures from 52 to 82 C. The pH was kept in the range 10 to 12 atall times by the addition of 29% NH OH. A vigorous reaction took placewith the evolution of hydrogen gas.

A more thorough understanding of our invention can be gained through thefollowing example illustrating the preparation of a 75 liter batch.

Seventy-five liters of water was heated to a temperature of 62 degreescentigrade, being vigorously stirred before, during and after theautocatalytic decomposition reaction by means of five three-bladedpolyethylene propellertype, motor-driven stirrers. To adjust the pH,0.75 liter of aqueous 29% ammonium hydroxide was added. After adjustingthe pH 2.24 kg. of tetrasodium pyrophosphate was dissolved in the hotwater and the pH again adjusted to fall in the range of 11 to 12 byadding 0.75 liter of aqueous 29% ammonium hydroxide. Then 75 grams ofmolybdenum trioxide are dissolved together with 1.88 kg.

0f nickelous sulfate was then dissolved in the hot solution and the pHagain adjusted to fall in the range of 11 to 12 by adding 3 liters ofaqueous 29% ammonium hydroxide. Then, 0.15 kg. of silver sulfate wasdissolved in the hot solution. At this point the solution was clear andhad a bluish-green color; 1.88 kg. of sodium hypophosphite was thenadded. In approximately one or two minutes the evolution of hydrogenbegan. Two minutes after the addition of the sodium hypophosphite,another 6.3 liters of aqueous 29% ammonium hydroxide was added. The pHwas now in the range of 11 to 12.

After about another four minutes the autocatalytic decompositionreaction became very vigorous and a large amount of nascent hydrogen wasliberated from the solution. The solution changed color frombluish-green to black because of the formation of the flocculent blacknickel-silver-phosphorus-Mo product.

When the reaction began to subside, an additional 223.8 grams oftetrasodium pyrophosphate and an additional 187.5 grams of sodiumhypophosphite was added. This assured completeness of the reaction andcomplete depletion of the reducible nickel and silver ion content of thesolution. The autocatalytic decomposition reaction was complete afterabout twenty minutes.

It is very important that during these twenty minutes the pH ismaintained in the range of 11 to 12 by adding an excess of aqueous 29%ammonium hydroxide. The total amount of aqueous 29% ammonium hydroxideadded is equal to about 14% of the total hot water volume. The productwas a finely divided, high surface area powder which contains about 53%nickel, 22% silver, 1% phosphorus and 7% molybdenum producing about 460grams of the powdery nickel, silver, molybdenum and nickelphosphorusal-loy product.

The precipitate of catalyst material is collected and washed with water.The phosphorus content of this catalytic material is variable. Usually,the phosphorus content of the catalyst product is within the range ofabout 0.6 to 2.24% phosphorus by weight, the remainder being molybdenum,nickel, and silver. The silver can vary from about 6.75 to 25.4% byweight, the nickel can vary from about 43.6 to 78.4% by weight, and themolybdenum can vary from about 6 to 8.5% by weight.

It will be apparent to one skilled in the art that the concentrations ofthe various starting materials can be varied by experimentation toproduce a composition having percentages within these ranges. Thespecific surface area of our catalyst prior to fabricating, also isvariable within the range of 6.7 to somewhat in excess of about 25 ,7meters 2/ gram.

The washed catalytic material is now ready to be shaped into a formsuitable for installation into a fuel cell. Most fuel cell designsrequire the electrode to assume the form of a thin sheet. Other fuelcells require forms such as cylinders which too can be satisfactorilymade from our material. The ultimate shape of the electrode depends noton our catalytic material but rather on the design of the individualcell.

Any suitable fabricating technique can be used to stabilize our catalystin the desired shape, even sintering provided the catalytic material isnot heated to a temperature in excess of 825 to 850 C. Heating to ahigher temperature causes the nickel-phosphorus phases present as alloysto begin to melt. Melting results in a decreased surface area.

Other suitable means for forming our catalyst product into a fuel cellelectrode include mixing it with a binding agent such as a thermoplasticresin; for example, polytetrafluoroethylene. The catalyst resin mixtureis then shaped and heated until the thermoplastic material softensslightly to thereby stabilize the electrode.

An especially desirable means for stabilizing the catalytic materialinto an electrode shape was found to be as follows. The catalyticmaterial was mixed with a small amount 9: fi $l9 fiber in an aqueousslurry; for example, 40 grams of catalyst to 0.5 gram asbestos fiber in200 ml. water.

The catalyst asbestos mixture was then poured into a Buchner funnelfitted with filter paper. The water was removed by suction, leaving adamp matte. Care was taken to assure a uniform thickness of the matte.

The matte together with the filter paper was removed from the Buchnerfunnel and a nickel support screen pressed lightly into the matte. Thisassembly was dried in a partial vacuum (25 mm. Hg) at 60 C.

When completely dry, the electrode was trimmed to size and installed asthe anode in a fuel cell operated at a reactant supply pressure of 18p.s.i.g.; hydrogen fuel; oxygen oxidant, a silver cathode; an aqueouspotassium hydroxide electrolyte concentration between 35 and 40% KOH byweight and a temperature of about 90 C. At 100 amps/ft. the cell voltagevaried between 0.81 and 0.86 volt.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. The method of producing a catalytic fuel cell electrode comprisingthe steps of preparing a solution consisting essentially of a dissolvednickelous salt and a dissolved silver salt having a silver ionconcentration not exceeding 0.55 mole Ag+ per mole of Ni++, a molybdenumsalt in which molybdenum is in an oxidation state of from +2 to +6 in aquantity sufficient so that the finished catalytic material containsfrom 6 to 8.5% molybdenum by weight, a dissolved alkali pyrophosphatehaving at least 1.00 mole of P O for each 1.45 moles combined total ofAg+ and Ni; a dissolved alkali hypophosphite having in excess of 2.75moles of (H PO for each 1.00 mole of Ni++; and maintaining the pH ofsaid solution in excess of pH 10 through addition of ammonium hydroxide;reacting said solution with the hypophosphite of said alkalihypophosphite thereby reducing said nickelous, molybdenum, silver ionsto a flocculant precipitate of catalytic material, said catalyticmaterial consisting essentially of nickel, molybdenum, silver andnickel-phosphorus alloy; collecting the catalytic material; andfabricating the catalytic material into a fuel cell electrode.

2. The method of producing a catalytic fuel cell electrode comprisingthe steps of preparing an aqueous solution consisting essentially of adissolved nickelous salt and a dissolved silver salt having a silver ionconcentration not exceeding 0.55 mole Ag+ per mole of Ni++, a molybdenumsalt in which molybdenum is in an oxidation state of from +2 to +6 in aquantity sufiicient so that the finished catalytic material containsfrom 6 to 8.5% molybdenum by weight, an alkali hypophosphite, and acondensed nickel complexing polyphosphate in a quantity suflicient toprevent precipitation of basic nickel salts; maintaining the pH of saidsolution in excess of 10; reacting said silver and nickelous ions withthe hypophosphite of said alkali hypophosphite; reducing said silver,molybdenum and nickelous ion to a flocculant precipitate of catalyticmaterial consisting essentially of molybdenum, nickel, silver, andnickel-phosphorus alloy; collecting the catalytic material; andfabricating the catalytic material into a fuel cell electrode.

3. The method of producing a catalytic fuel cell electrode comprisingthe steps of preparing an aqueous nickelous and silver ion solution byadmixing water, a nickelous salt, a silver salt, a molybdenum salt inwhich molybdenum is in an oxidation state of from +2 to +6 in a quantitysuflicient so that the finished catalytic material consists of from 6 to8.5% molybdenum by weight, an alkali hypophosphite, a condensed nickelcomplexing polyphosphate in a quantity sufficient to preventprecipitation of basic nickel salts; and suflicient hydroxide selectedfrom the group consisting of the alkali and amonium hydroxides tomaintain the pH of said solution between 10 and 12; reacting saidmolybdenum, nickelous, and silver ions with the hypophosphite of saidalkali hypophosphite, reducing said molybdenum, nickelous, and silverions to a flocculant precipitate of catalytic material, said catalyticmaterial consisting essentially of molybdenum and nickel-phosphorusalloy; collecting the catalytic material; and fabricating the catalyticmaterial into a fuel cell electrode.

4. A fuel cell electrode comprising a catalyst consisting essentially offrom about 6.75 to 25.4 weight percent silver, from about 6.0 to 8.5weight percent molybdenum, from about 43.6 to 78.4 weight percent nickeland from about 0.6 to 2.24 weight percent phosphorus.

5. A fuel cell having a housing; two electrodes mounted in said housingin spaced relation to each other; means to supply a fuel to one of saidelectrodes; means to supply an oxidant to the other of said electrodes;an electrolyte disposed between said electrodes; and at least one ofsaid electrodes consisting of an electrically conductive porous supporthaving randomly dispersed thereover and in the pores thereof a catalystconsisting essentially of from about 6.75 to 25.4 weight percent silver,from about 6.0 to 8.5 weight percent molybdenum, from about 43.6 to 78.4weight percent nickel and from about 0.6 to 2.24 weight percentphosphorus.

No references cited.

ALLEN B. CURTIS, Primary Examiner.

WIN STON A. DOUGLAS, Examiner.

A. SKA-PARS, Assistant Examiner.

1. THE METHOD OF PRODUCING A CATALYTIC FUEL CELL ELECTRODE COMPRISINGTHE STEPS OF PREPARING A SOLUTION CONSISTING ESSENTIALLY OF A DISSOLVEDNICKELOUS SALT AND A DISSOLVED SILVER SALT HAVING A SILVER IONCONCENTRATION NOT EXCEEDING 0.55 MOLE AG+ PER MOLE OF NI++, A MOLYBDENUMSALT IN WHICH MOLYBDENUM IS IN AN OXIDATION STATE OF FROM +2 TO +6 IN AQUANTITY SUFFICIENT SO THAT THE FINISHED CATALYTIC MATERIAL CONTAINSFROM 6 TO 8.5% MOLYBDENUM BY WEIGHT, A DISSOLVED ALKALI PYROPHOSPHATEHAVING AT LEAST 1.00 MOLE OF P2O7-- FOR EACH 1.45 MOLES COMBINED TOTALOF AG+ AND NI++; A DISSOLVED ALKALI HYPOPHOSPHITE HAVING IN EXCESS OF2.75 MOLES OF (H2PO2)FOR EACH 1.00 MOLE OF NI++; AND MAINTAINING THE PHOF SAID SOLUTION IN EXCESS OF PH 10 THROUGH ADDITION OF AMMONIUMHYDROXIDE; REACTING SAI SOLUTION WITH THE HYPOPHOSPHITE OF SAID ALKALIHYPOPHOSPHITE THEREBY REDUCING SAID NICHELOUS, MOLYBDENUM, SILVER IONSTO A FLOCCULANT PRECIPITATE OF CATALYTIC MATERIAL, SAID CATALYTICMATERIAL CONSISTING ESSENTIALLY OF NICKEL, MOLYBDENUM, SILVER ANDNICKEL-PHOSPHORUS ALLOY; COLLECTING THE CATALYTIC MATERIAL; ANDFABRICATING THE CATALYTIC MATERIAL INTO A FUEL CELL ELECTRODES.
 4. AFUEL CELL ELECTRODE COMPRISING A CATALYST CONSISTING ESSENTIALLY OF FROMABOUT 6.75 TO 25.4 WEIGHT PERCENT SILVER, FROM ABOUT 6.0 TO 8.5 WEIGHTPERCENT MOLYBDENUM, FROM ABOUT 43.6 TO 78.4 WEIGHT PERCENT NICKEL ANDFROM ABOUT 0.6 TO 2.24 WEIGHT PERCENT PHOSPHORUS.